Abstract - Drought hardening, soluble carbohydrates and free amino acid accumulation in Acacia cyanophylla Lindl phyllodes. Two plots of 8-month-old plants of Acacia cyanophylla Lindl grown in the greenhouse have been subjected to two contrasting water feedings. The first was nearly maximum evapotranspiration (D) and the second was D/6. Three plants of each water feeding were subjected to a drying cycle by water withholding after 1, 3, 5 and 13 months in order to allow the emergence of adaptive mechanisms in hard drought. At the beginning of each drying cycle, water potential component had been measured at 1 Pm in order to evaluate the effect of drought preconditioning duration on the D/6 plants. At the end of each cycle, marked by zero turgor, the same measures had been made and the remaining phyllodes were used to determine the concentration of free amino acids, soluble sugars and starch. We noted at the beginning of the cycles that D plants had a slight decrease in osmotic potential and a slight increase in the turgor; these variations seemed to be related to age. The response of D/6 plants varied with the duration of preconditioning. After 1 month, at the beginning of the cycle, their turgor was less than that of D plants. At the end of the cycle, they had a less negative osmotic potential and lower contents of free amino acids as compared to D plants. This suggests that a 1 month preconditioning to D/6 treatment was insufficient to induce hardening mechanisms. After 3 months and at the beginning of the cycles, we noted a higher turgor and a lower osmotic potential in the D/6 plants with an increasing difference in osmotic potential between D and D/6 over time (from 0.15 to 0.47 MPa). At the end of the fourth cycle, D/6 plants maintained a positive turgor at water potentials near -4.50 MPa. The difference of osmotic potential between D and D/6 plants was significant (from 0.40 to 1.60 MPa) and was accompanied by an increasing accumulation of soluble sugars and free amino acids, especially proline. Concomitantly, a reduction in starch concentration was observed. This could in part explain the accumulation of soluble sugars. These results attest to an acquired hardening by D/6 plants subsequent to a preconditioning of more than 3 months. This hardening was expressed at the end of the drying cycles by increasing osmotic adjustment ability over time.